1. Field of the Invention
This invention relates to the treatment of organic polymer surfaces with antireflection coatings. More specifically, this invention relates to a composite structure that includes an organic polymer substrate film having on one or both of its surfaces at least one coating which reduces light reflection and increases light transmission. The coatings of this invention are characterized as causing relatively small amounts of coloration and haze.
2. Description of Prior Art
One method for reducing light reflection from a substrate (such as a polymer surface) is to coat the surface with an antireflective layer having a thickness of about a quarter wavelength. The antireflective layer may be a second polymer, or an inorganic material such as a metal fluoride, metal oxide, or metal nitride, where the deposited layer has a refractive index less than that of the substrate. A method for producing such a layer is disclosed in U.S. Pat. No. 4,066,814. If the deposited antireflection layer is in contact with the air, the maximum reduction in reflection is achieved when the reflective index of the deposited antireflective layer equals the square root of the refractive index of the substrate. This approach has two limitations. First; single layer, low refractive index, antireflection coatings have substantial reflected and transmitted coloration. Secondly, the low refractive index inorganic materials typically used in these antireflection coatings, for example magnesium fluoride, silicon dioxide, or cryolite, are often deposited by evaporative techniques, not by dc magnetron sputtering, a popular method for accurately coating polymer substrates.
Antireflection coatings having wider bandwidths (and consequently less coloration) may also be obtained by using multiple deposited layers. As described in Optical Thin Films User's Handbook by James D. Rancourt, Macmillan Publishing Company, 1987, two common antireflective coating designs are the quarter-quarter and the quarter-half-quarter stacks. That is to say, the reflective coating is made up of a number of layers having differing refractive index materials, each equal in thickness to one-quarter or one-half of a wavelength sought to be antireflected. The applicability of these designs to dc magnetron-sputtering is restricted due to materials availability and to manufacturing complexity.
Another way to decrease the reflection of a substrate surface is to include a porous coating. As taught in U.S. Pat. No. 4,252,843, porous coatings which result in a graded refractive index are particularly effective antireflection coatings. However, there are very few commercially viable processes for producing such coatings.